Technological Field
The present invention relates to a paper thickness detecting device and an image forming apparatus including the paper thickness detecting device.
Description of the Related art
Some image forming apparatuses such as copying machines and printers have a paper thickness detecting device for detecting the thickness of a paper including a document. Paper thickness detecting devices detect the thickness of a paper for example from the displacement of a roller when the paper passes through a nip part of a pair of rollers that conveys the paper. By using a paper thickness detecting device, it is possible to control the paper conveyance speed or the fixing temperature according to the paper thickness, thereby enabling improving the image quality.
Moreover, in a highly functional image forming apparatuses such as a multifunction peripheral (MFP), control is performed to enable output of preferable image quality by optimizing parameters such as the fixing temperature and transfer conditions with respect to the basis weight, the heat capacity, and the surface resistivity of a paper to be printed. Therefore, it is necessary that a user selects the paper type at the time of printing and inputs information to the MFP what type of paper is to be printed.
However, since it is very difficult for most of general users to distinguish the paper type, it is not always the case that an appropriate paper type is set.
Therefore, in recent years, it has been proposed that MFPs have a function of automatically discriminating the paper type and automatically set the optimum paper type. There are various methods for discrimination of the paper type. For example, there are methods of discriminating the paper type by utilizing the fact that the basis weight and the heat capacity are highly correlated with the paper thickness and collating data obtained by actually measuring the paper thickness with a paper thickness database of each paper type.
This paper type discrimination by the paper thickness has an advantage that the paper thickness can be measured accurately by providing a simple paper thickness measuring mechanism in a paper passing path.
Among these methods, especially a paper thickness detection method of pinching a paper by a pair of detection rollers and reading the amount of displacement by a displacement sensor using one of the rollers as a reference has a simple structure and can be incorporated in a conventional conveyance path with low cost and thus is widely used.
However, although this paper thickness detection method has a simple structure, one of the pair of detection rollers is a driving roller, and thus vibration occurs in the vertical direction with respect to a gear axis, which is transmitted to the pair of detection rollers, and thus the amount of displacement tends to fluctuate. This may affect the accuracy of measurement of the paper thickness. In particular, the source of vibration is often located at an end part of a roller shaft. Vibration fluctuates depending on the distance between the source of vibration and the detection rollers, which affects the accuracy of paper thickness detection.
Thus proposed is a method of calculating the thickness of a sheet material on the basis of detection data when conveyance of the sheet material is stopped while the sheet material is interposed and data of rollers before interposing the sheet material in the same phase (see, for example, JP 2002-187641 A).
In a method described in JP 2002-187641 A, since conveyance of a paper is stopped in order to detect the paper thickness with high accuracy, there is a problem that the productivity is deteriorated.
In order to improve the accuracy of paper thickness detection without deteriorating the productivity, it is conceivable to increase the pressure between the pair of detection rollers to suppress the vibration; however, this may disadvantageously deteriorate the performance of passing a paper. Moreover, a stable measurement value of the paper thickness can be obtained by allowing the pair of detection rollers to follow; however, the conveyance resistance at the time of entry of a paper into a nip part of the pair of detection rollers becomes large. This results in a problem that it is difficult to pass a thin paper having low rigidity or other types of paper.